How Important are Offset and Backspacing?

Another example of helpful factory wheel markings is illustrated on this 15x10 Pro Comp 1079 wheel that has a negative 46mm offset.

Another example of helpful factory wheel markings is illustrated on this 15x10 Pro Comp 10

Speaking of aftermarket wheels, companies like Dick Cepek and Mickey Thompson work closely with the suspension companies when they design their wheels. Sneddon says it's not unusual for them to test-fit a number of wheels on a new suspension kit to ensure everything fits correctly. "We do a lot of test fitting, making sure our wheels fit different models, and we're updating our application guide all the time," he says.

So how important is offset and backspacing? Well, it's very important in the world of four-wheel drive modification. Fortunately, you don't have to "go it alone," thanks to the testing and expertise of the wheel and suspension manufacturers.

How To Measure Backspacing
In the four-wheel drive world, backspacing is referred to far more often than offset. Look at most suspension kit specifications and you'll see they recommend a specific wheel size with a specific backspacing. Offset is rarely mentioned, and backspacing is one of the main components used for wheel selection. It's very easy to determine your wheel's backspacing. As shown in this photo using a stock Jeep TJ factory wheel, place the wheel (without tire) face-down and rest a ruler horizontally on the wheel. Now measure the distance between the hub mounting surface of the wheel and the ruler. That figure is your backspacing.

Spaced Out
Wheel spacers can also help to create clearance. There are many sources for spacers, and they are available in a variety of thicknesses. Most are made from high-grade aluminum and have weight and tire size ratings. Adding wheel spacers will have the effect of running a wheel with a negative offset, and they create space for larger tires. The downside to wheel spacers is that they can increase stress on axle componentry just like a negative offset wheel. Also remember that because the spacers bolt onto the hub and the wheel bolts to the spacer, you now have two sets of fasteners to torque and re-torque.

When purchasing wheel spacers, carefully measure things out ahead of time so that you can purchase spacers that are no thicker than they have to be.